1. Field of the Invention
The present invention relates to devices for performing an osteotomy. More particularly, preferred embodiments of the internal osteotomy fixation device cause minimal deformation of surrounding tissue and skin, are easy to implant, and are easy to manipulate after implantation.
2. Description of the Related Art
A high tibial osteotomy (HTO) is a procedure to correct misalignment of a lower limb due to the wear of cartilage in the knee or bone deformities. A HTO is usually performed to relieve pain from osteoarthritis. The procedure involves making an incision from the medial side of the upper tibia all the way across to the cortex on the lateral side. The incision is then opened into a wedge shape to change the alignment of the joint. An implant device holds the wedge open until it is filled in by new bone growth.
The HTO restores the proper anatomic and mechanical axes of the extremity. The operation is usually performed adjacent to the end of the tibia, without removing the articular (end) surface, as would be the case for preparation of the tibia for implantation of a joint prosthesis.
The angle correction necessary to achieve the proper reorientation is an important consideration in each HTO procedure. Determining this angle prior to or during the HTO procedure is often difficult. Therefore, the ability to adjust the implant device during the implantation procedure, and after the implantation procedure is complete, is very advantageous. Preferably such adjustment does not require further incisions or other invasions of the patient's body. Further, the tibia heals faster and causes less discomfort for the patient when the tibial incision is widened several times in small increments. Thus, prior art devices provide the capability to incrementally adjust the tibial incision using external adjustment means.
U.S. Pat. No. 5,827,286 (the '286 patent) discloses an incrementally adjustable tibial osteotomy fixation device and method. The device comprises a first plate member and a second plate member telescopically received within the first plate member. A proximal end of the first plate member includes a ratchet arm that extends toward a distal end of the first plate member. Teeth at a distal end of the ratchet arm cooperate with grooves on a face of the second plate member to allow advancement of the second plate member in a first direction. Lands on the opposite side of the teeth preclude movement of the second plate member in the opposite direction. Thus, the length of the fixation device can only be extended and not retracted. Both the first and second plates include screw holes through which bone screws extend in order to attach the plates to a tibia. The second plate member includes a bend near the proximal end that represents the general contour of the tibia.
When the device of the '286 patent is implanted, it is in its fully contracted position. The first plate member is secured to the tibia distal the tibial incision, while the second plate member is secured to the tibia proximal to the tibial incision. After completion of the implantation procedure, the wound and incision are allowed to heal for a few days to a few weeks. After proper healing, the physician gradually opens the tibial incision, thereby adjusting the angular orientation of the patient's tibia with respect to the rest of the leg.
To open the tibial incision, the physician incrementally lengthens the implant device. To incrementally lengthen the implant device, the physician places the patient in an external extension brace, which is illustrated in FIGS. 10A and 10B. The brace comprises three pads. A first pad is located on a first side of the patient's leg, which is the same side on which the device is located, proximal the device. The second pad is located on the same side of the patient's leg, distal the device. The third pad is located on the opposite side of the patient's leg, at about the same position as the device.
The first and second pads are stationary. The third pad is movable toward the patient's leg. With all three pads contacting the patient's leg, the physician urges the third pad further toward the patient's leg, thereby applying a bending load to the leg. The device is located at or near the point of maximum deflection, on the side of the leg that is placed in tension by the deflection. The tension widens the tibial incision and lengthens the device. When the physician removes the bending load from the patient's leg, the interaction of the ratchet teeth on the first plate member and the grooves on the second plate member prevents the device from contracting, or shortening in length. After allowing the incision to heal, the physician again lengthens the device in the same manner until the patient's leg reaches the proper angular alignment.
The device of the '286 patent, however, is not well suited for use in an osteotomy procedure. First, the device only allows for one degree of freedom, i.e. longitudinal extension. Thus, the device offers little flexibility in initially fitting the device to the patient's tibia. Furthermore, opening a tibial wedge creates motion in more than one direction. Not only do the proximal and distal edges of the wedge move apart, but the angular alignment of the proximal and distal bone segments changes. As the wedge is opened farther and farther, the device is not able to conform to the geometry of the opening wedge. The device thus creates stresses in the tibia that could lead to fracture.
Second, the device of the '286 patent locates the ratchet mechanism on the medial side of the implant. Therefore, the ratchet arm is stacked on top of the plate member including the grooves, which is in turn stacked on top of the tibia. The device thus protrudes a significant distance from the bone, causing unsightly deformation of the overlying skin and irritation to the patient.
Third, the configuration of the ratchet mechanism of the '286 device causes the device to be unnecessarily long. The ratchet design is essentially a lever spring with teeth at one end. The lever spring has a finite length, L1. This length must be sufficient to impart enough flexibility to the ratchet to allow the teeth to pass over the grooves. The length, L2, from the first, or most proximal groove, to the last, or most distal groove, determines the range of extension of the device. The ratchet teeth are distal to the ratchet. When the device is implanted, the entire groove portion is distal to the ratchet. Thus, the minimum total device length is the sum of L1 and L2. The device length could be reduced if the device could be arranged so that the minimum device length were equal to the greater of L1 and L2, rather than their sum.
Fourth, the ratchet arm is attached to the first plate member as a simple cantilevered beam. Thus, loads tending to retract the device are primarily shouldered by the ratchet arm. Intense loads could cause the arm to fail, forcing the patient to have to undergo the HTO procedure all over again. To avoid such a catastrophe, the ratchet arm instead must be built sturdily enough to handle these intense loads. Generally, however, the sturdier the arm must be, the larger it must be. And the larger the arm is, the more it causes unsightly deformation of the patient's skin and patient irritation.
Fifth, the ratchet arm has a simple rectangular cross-section along its entire length. Thus, because it is a cantilevered beam, during flexion the maximum stress in the ratchet arm occurs at the point where the ratchet arm is attached to the first plate member. The remainder of the arm's length is under relatively little stress. Therefore, the flexibility of the arm is limited, because relatively little deflection could cause the arm to undergo plastic deformation at the point of maximum stress. If the arm deforms plastically, its usefulness as a ratchet is severely diminished.
Sixth, the device of the '286 patent is rather difficult to retract after it has been implanted. The physician must make an incision in the patient's leg just distal of the distal end of the ratchet arm. He then inserts a tool similar to a crowbar and pries the ratchet away from the grooves. First, the incision that the physician must make in order to insert the crowbar is rather large. Second, the physician has to blindly feel his way through the incision to find the proper place on the ratchet to engage the end of the crowbar. Both of these features of the device are likely to cause unnecessary trauma to the patient's leg. Third, after the physician pries the device open with the crowbar, he or she must hold the crowbar in place with one hand while retracting the implant device with the other hand. This procedure requires a great deal of dexterity. If the crowbar slips, the patient may suffer further unnecessary trauma.
Seventh, the second plate member of the '286 device is attached to the proximal bone segment with only two bone screws inserted through two holes on the plate member. As the tibial incision is widened, the healing bone tissue within the incision is stretched. Thus, it tends to pull the incision closed. Each of the screws through the second plate member must shoulder half of the load imposed by the healing tissue. Not only is there a risk that the screws could pull out, but the stress concentrations in each location cause trauma to the bone tissue.
Therefore, an osteotomy fixation device that eliminates the negative features outlined above would be of great value to physicians and patients undergoing osteotomy procedures.